The several types of neoplastic transformations are accompanied by alterations in the composition of cell glycoproteins, which are major structural components of cell surfaces. One such observed alteration is in the level of sialic acid on the cell surface. In the present investigation, plasma sialic acid levels were measured in normal volunteers and neoplastic patients using thiobarbituric acid spectrophotometric methods. The mean plasma sialic acid level from 124 normal volunteers was 3.0 mumol/ml. The mean for 20 non-malignant patients was 3.2 mumol/ml. Such observed mean values of sialic acid were 3.7 mumol/ml in 64 breast cancer patients, 5.1 mumol/ml in 22 lung cancer patients, 4.1 mumol/ml in 20 colon patients, and 5.0 mumol/ml in 26 patients having ovarian, cervix, pancreas, prostate, thyroid, uterine, squamous cell, esophageal and endometrial cancers. Serial determinations of plasma sialic acid in 15 patients correlated well with the progression and regression of disease. These results indicate that plasma sialic acid levels are elevated over control levels in the different types of cancer patients studied. Assay of plasma sialic acid is not sensitive enough to be used for screening, but could be used as a prognostic determinant in a variety of neoplastic conditions.